Electrophotographic printers with thermal print fixing are generally known. When using thermal printing technology, the recording medium and the toner image located on the recording medium are conducted between two rotating rollers which touch one another under pressure. In this arrangement, one of the rollers, the so-called fixing roller, is heated. In order to achieve an adequate adhesion of the toner image on the paper by fixing, it is necessary for the toner particles in the fixing station to be heated beyond a melting point and to merge and, in addition, for the melted toner particles to be bonded to the paper structure.
With increasing printing speed and the thus increased transport speed of the recording medium, the heating time of the recording medium, and thus the achievable fixing temperature on the recording medium, is reduced. At higher transport speeds of the recording medium, the required fixing temperature can no longer be reached without additional heating of the recording medium, in particular if it is a recording medium with high basis weights.
In the electrophotographic printers which process reel paper, this is realized by means of a preheated paper guide saddle via which the reel paper web is guided with its rear and preheated before the actual fixing. A thermal print fixing device of this kind with preheating saddle is known from U.S. Pat. No. 4,147,922.
This method has the disadvantage that there may not be print already fixed on the rear of the reel paper web since it would otherwise be etched by the high preheating temperature and destroyed. A modular operation of two printers, the first printing the front side and the second printing the rear, is thus not possible. The same also applies to a front and rear print by one and the same printer in which the front side is printed during the first run through the stack and the rear is printed during the second run through the stack.
In order also to be able to process toner with a high melting point at a high fixing speed, a heat melting fixing device for toner images located on a recording medium is known from German reference DE-PS 27 17 260, said device having a preheating saddle which is pivotably mounted in the direction of the fixing roller. Thus, the wrapping angle of the recording medium on the fixing roller can be set.
However, this heat melting fixing device also requires the existence of a heated preheating saddle as an absolute necessity for optimum heat melting fixing.